I. Field of the Invention
This invention relates generally to heating, ventilating and air conditioning (HVAC) systems for public buildings. More specifically, it relates to a programmable device for controlling HVAC systems in a way that optimizes energy use consistent with the power company's price schedule to reduce the energy costs associated with operating the building.
In recent years, energy management in commercial buildings has become a growing concern for building owners, building tenants and electric companies alike. Building owners and tenants, troubled by rising energy costs, have looked for new ways to cut consumption. Similarly, electric companies, unsure of their ability to keep up with the rising demand, have begun to promote more sophisticated energy management systems for commercial building applications.
Many electric companies have adopted a strategy under which peak electric consumption would be shifted to non peak hours, thus, reducing peak demand. Pricing incentives have been adopted in accordance with this strategy by the suppliers of electricity. By successfully shifting consumption patterns to reduce peak demand, power companies are able to reduce their generation capacities. This, in turn, reduces the capital expenditures required of the power company for electrical generating equipment.
In order to reduce peak demand, energy companies have also actively promoted the use of cool storage systems by offering installation and rate incentives. Such cool storage systems are being installed in many new commercial buildings as well as in existing supermarkets, restaurants and office buildings. When installed in either a new or an existing building, cool storage systems operate by storing cooling energy in the form of ice or chilled water at night or during other off-peak electrical rate periods. The stored cooling energy is then used the following day during peak electrical rate periods to meet the buildings' cooling load.
Storing cooling energy at night for use during peak electric rare periods not only reduces the buildings' initial electricity demand, but also saves additional money due to the differential between off-peak and peak energy rates. Such savings, of course, vary according to the building's load profile, storage system size, control system and utility rates. The programmable device of the present invention takes these and other factors into account to optimize reductions in electricity costs.
II. Description of the Prior Art
In the past, heating and cooling of large buildings has normally been accomplished by circulating conditioned air through ventilating ducts that extend throughout the building. As discussed in U.S. Pat. No. 4,513,574 which issued on Apr. 30, 1985 to Humphries, et al. the air used to cool the building is normally supplied at about 55 degrees fahrenheit. In such systems, either the ducts or the air diffusers which discharge the conditioned air into the rooms of the building are equipped with flow control devices to permit each room to be controlled individually. While individual room control does result in lower energy consumption, such systems typically do not have the ability to store cooling energy for later use. Hence, the buildings, peak energy consumption periods typically matches the period of time during which the power company charges its highest rate.
More recently, systems have been developed which take advantage of off-peak energy rates. These systems achieve additional economies by using outside air in cool weather and cooling at night to precool the building mass. Many such systems also use ice or cold water storage for storing cooling energy. In such systems, refrigeration machines are operated at night in hot weather to precool building slabs and to make ice or chill water in storage tanks. This is done when the building is virtually unoccupied and the lights are off. Then, when cooling demand increases during the day, the pre-cooling of the building mass delays the need for peak mechanical cooling. When additional cooling is required, cold water or slush is circulated between the storage tanks and a secondary cooling coil in the air conditioning system to provide the necessary peak cooling in the afternoon. The storage in the building mass and the ice tank together work to keep the building cool during demand peaks and when the power rates are highest. The intent of such systems is to help avoid high peak demand charges by reducing electrical consumption during peak rate periods.
While cold storage systems have proven to be a reliable means for reducing total energy consumption in the building, the control units for such systems have been relatively unsophisticated. Conventional control techniques typically use a time sequence that relies on a pre-programmed chiller schedule. These controllers typically have been unable to take into account climatic fluctuations and, therefore, have only very imprecisely calculated the required storage amount to reduce electrical demand during peak periods. As a result, some days storage is completely depleted before the peak period has ended. The building must then rely on its chiller for direct cooling, resulting in high demand charges. Conversely, on days when the cooling load is low, storage is not effectively utilized since the chiller comes on according to a preprogrammed schedule. As a result, the system builds up too much ice in storage. This ice simply goes to waste. In either event, the building's electric bill is needlessly increased.